Among the proposals in the Welsh government's new environment bill is the introduction of a minimum charge for bags for life if demand continues to rise. They are currently exempt from the 5p minimum charge rules.

Ministers have suggested that the rise in sales of bags for life could be having a detrimental effect on the environment if they are being disposed of incorrectly, as the thicker gauge of plastic takes longer to degrade.

But Wrap data also suggests there has been a 120-130% increase in sales of bags for life during the same period, and ministers suspect the heavy duty bags are now being used as "throwaway" substitutes for single-use bags.

The Welsh government has recorded bags being sold by supermarkets for as little as 5p to 10p, and it believes introducing a minimum charge could tackle the increase.

It said the increase in the sale of bags for life is currently still within the levels predicted in its initial impact assessment, but ministers are calling for powers to bring in minimum charges, "should future evidence suggest this is necessary".

In the white paper for the environment bill, the government said: "An increase in demand for bags for life was expected in Wales following the introduction of the charge, as consumers got used to reusing their bags and purchased enough bags for life for their shopping needs. However, there is a potential risk that many consumers start treating a low-cost reusable plastic bag as a 'throwaway bag' and discard it prematurely.

"This is likely to have significant adverse environmental impacts if these types of bags are disposed of inappropriately; as such bags are typically made of heavier gauge materials and take longer to degrade."

A spokesperson for the Department for Environment, Food and Rural Affairs (Defra) said the minimum charge policy in England would initially cover single gauge bags only, but added that the department would monitor the situation in Wales and other countries.